THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 



occurring in the adjacent deposits. However, it is very possible and not 

 uncommon that deposits in such channels would be made up of material 

 carried from a considerable distance and both the inorganic and organic 

 contents might be derived from regions remote both in space and character 

 from the observed banks of the old stream. A stream descending from a 

 mountain or a high plateau to a low r land might sweep down and deposit 

 on the flat material of a totally different composition and the remains of a 

 life belonging to zones of radically different temperatures, altitudes, and 

 soils. It is possible that remains of animals of very different habitat might 

 be found embedded together. 



The author has in mind a locality in New Mexico where such a fossil 

 stream-channel yielded the skull of a highly aquatic amphibian in close 

 association with the vertebra of a land reptile, while but a few feet away 

 in the red sandstones and clay which were once the margin of the stream 

 remains of terrestrial and swamp animals occurred in fairly regular beds. 



The presence of stream-channels, except in the rare instances of under- 

 ground streams, is an evidence of subaerial erosion, and they must uniformly 

 lie below the level of the lower plain of an unconformity. There may be a 

 bending down of the line of unconformity at this point if the old stream- 

 valley was a wide one or had cut deeply into the older rocks. In cases where 

 the river deposits were slight in amount or where the interval of exposure 

 was long with much erosion, or where marine planation had followed, all 

 traces of the river might be removed. 



Flood-plain deposits are not infrequent. 1 The flooding of streams from 

 whatever cause results in accumulations of material in wide flats which 

 may attain considerable thickness. The fluctuation of currents in the 

 flood waters and the very position and duration of the deposition causes 

 the beds to be extremely irregular in arrangement and composition. Rapid 

 variations of the strike and dip of outcrops, prevalent discontinuity of 

 individual beds, cross-bedding, and truncation of older beds by younger, are 

 common characters. Conglomerates, sandstones, shales, and muds alternate 

 rapidly. Such deposits yield a pretty full record of their history upon close 

 examination. 



The physical characters of flood-plain deposits formed in arid and 

 humid regions would be in many ways very similar, though it is probable 

 that the violence of the floods of arid regions would leave their record in the 

 coarser material and the evidence of stronger currents. 



The chemical and physical character of the material reveal to a large 

 extent the climatic conditions under which the beds were formed. Flat 

 or flood-plain deposits of arid regions are marked by the presence of highly 

 oxidized or carbonated minerals with a lack of hydrous oxides or sulphides, 



1 For an attempt to describe a flood-plain and subaerial delta region and reveal its history, 

 see Case, Carnegie Inst. Wash. Pub. No. 207, 1915. 



